Speed control for electric motors



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Y O Q f n u Feb. 8, 1949. w. H. ELLIOT SPEED CONTROL FOR ELECTRIC MOTORS Filed Oct. 26, 1944 Feb. 8, 1949. w. H. ELLIOT SPEED CONTROL FOR ELECTRIC MOTORS 2 Sheets-Sheet 2 Filed Oct. 26, 1944 Q Patented Feb. 8, 1949 SPEED CONTROL FOR ELECTRIC MOTORS William B. Elliot, Shorewood, was. minor to Cutler-Hammer, Inc., Milwaukee, Wis., a corporation of Delaware Application October 26, 1944, Serial No. 580,377

9 Claims. (Cl. 318-45) This invention relates to a system of automatic speed control over a wide range of speeds of electric motors and while not limited thereto is of particular advantage for the control of motor driven printing presses and the like which must be operated at accurately maintained speeds even though the load varies.

An object of the invention is to provide an automatic speed regulator which responds to.

tendencies toward variations in speed in their incipiency.

Another object is to provide a speed regulator which limits departure from a given speed upon variations in load to a substantially infinitesimal amount.

Another object is to provide a speed regulating system of the aforesaid character utilizing driving motors of minimum size. I

Another object is to provide a two motor driving system in which one motor is provided with regulating means which affords only a limited number of speed points within the range of speeds required, while a second smaller motor supplies supplemental power so as to afiord Joint operation of the two motors at any desired speed intermediate of any two speed points afforded by the regulator for the large motor.

Another object is to provide a two motor driving system in which the power delivered by one motor is limited to a small fraction of the power delivered by the large motor and in which variations in the load of the small motor are employed to effect control of the large motor.

Other objects and advantages will hereinafter appear.

The accompanying drawings are illustrative of two embodiments of the invention.

In the drawings, I

Fig. 1 is a diagram of connections of a system employing a main induction motor and an auxiliary direct current dynamo electric machine.

Fig. 2 is a diagram of the speed-torque characteristics of the machines of Fig. 1, while Fig. 3 is a connection diagram of a modification of the system illustrated in Fig. 1.

Referring to Fig. 1, the system is suppliedwith power from the bus bars Ll, L2 and L3 of a three phase power supply. The machine which is to be operated by the system is provided with a slip ring type main motor i, having a stationary primary winding i and an armature or secondary winding l coupled to the driving shaft of the machine. Coupled to this shaft is also the armature 2, of an auxiliary direct current motor 2. The motor 2 is provided with a 2 separately excited shunt field winding 2*, which may be supplied with direct current energy from a suitable source (not shown). The secondary winding l is connected to the star connected resistors l, I and 3 of a speed regulator 3. The resistors are provided with intermediate taps. each tap being connected to a contact button, the buttons of the different phases being arranged in parallel rows as shown. Said speed regulator is also provided with a cross-head 3 and. upon reciprocation, said cross-head successively engages the contact buttons of the corresponding resistors to successively cut in or out the diflerent steps of said resistors, to vary the speed of the motor I.

The cross-head I may be reciprocated by an engaging screw 3', which isattached to the shaft of a reversible pilot motor 4. The pilot motor 4 is a single phase motor provided with forward and reverse windings 4* and 4, respectively. One terminal of each of the two windings 4 and l is connected to the line L2, while the connection of the second terminal'of the two windings will be explained hereinafter. The regulator 3 is also provided with a limit switch 3', having normally closed contacts which are operable to open position when the cross-head 3 is in the extreme position where all of the resistors 3, 3 and 3 are inserted in circuit with the winding l The armature Z 'is supplied with rectified direct current through a transformer 5, having a primary winding 5*, connected across the lines LI and L2, and also a center tapped secondary winding 5*. The end terminals of winding 5 are respectively connected to the anodes 6 and I of gaseous electron tubes 6 and I, which are also provided with cathodes 6 and 1*, respectively, and control electrodes or grids 6 and 1, respectively.

A pilot controller, hereinafter described, is employed to regulate the flow of current through the gaseous electron tubes 8 and 1. It may be of any suitable type to control the instant at which the respective tubes become conducting during their positive half cycles, employing such well known methods as phase-shift" or amplitude control for modifying the grid potentials. The grid potentials of tubes 6 and I are so governed in relation to the armature voltage of the auxiliary motor, when compensated for IR drop, that a preset speed may be automatically maintained. The pilot controller preferably also includes means to limit the armature current of motor 2 to a preset maximum value under all conditions of operation.

The system further includes electromagnetic relay I, having an energizing winding 8, and

normally open contacts 9, an electromagnetic energizing winding I3, normally open main contacts I3, I3 and I3 and normally open auxiliary contacts I3, I3 and I3 and normally closed auxiliary contacts I3 A normally closed stop push button switch I4 and a normally open start push button switch I5 provide for manual initiation of starting and stopping of the system.

The pilot controller is constructed as follows: A source of constant voltage direct current sup plies the bus bars I8, I! and I8. the bus bar l6 being at zero potential, while the potentials of the bus bars I1 and I8 are plus 105 volts and plus 255 volts respectively, with respect to the potential of the bus bar I6. Connected across the bus bars I1 and I8, is a voltage dividing resistor I9, having a movable contact I9. A high vacuum electron tube 20, having a cathode 20 and an anode 20 and a control electrode 20, has its cathode 2i! connected to the bus bar it through a resistor 2i, while the anode is connected to the bus bar I8 through a resistor 22. A second high vacuum electron tube 23 provided with a cathode 23, and anode 23 and a control electrode 23, has its cathode connected through the contacts I3 to the bus bar I6, while its anode 23 is connected through a resistor 24 to the anode 20. A smoothing condenser 25 is connected between the bus bar I8 and the anodei23 The cathode 23 is also adapted to be connected to the bus bar I'I, through the normally open contacts I3 There is a further connection from the cathode 23, through a condenser 21 and a resistor 28 to the bus bar I6. A resistor 26 is interposed between bus bar I! and condenser 21. Connected across the terminals of the motor armature 2 is a voltage dividing resistor 29, the contact 29 of which is connected to the movable contact i9; Connected in series with the cathodes 6 and i respectively, are the primary windings 30 and 3i, of series transformers 30 and 3|, respectively. The secondary windings 30 and 3 I of said series transformers each have one terminal connected in series with a voltage dividing resistor 32, to the bus bar IS. The other terminals of said secondary windings 30 and 3i, are connected through rectlfiers 33 and 34, respectively, in series with a voltage dividing resistor 35, to the bus bar I6. A smoothing condenser 36 is connected in parallel with the voltage dividing resistor 32. The grid 20 is connected to the movable contact 32 of the voltage dividing resistor 32, while the grid 23 is connected to the movable contact 35 of voltage dividing resistor 35.

An alternating current component of the control voltage for the tubes 6 and I is supplied by a phase shifting network comprising a condenser 31, and a resistor 38, connected in series across the lines LI and L2. There is also connected across the lines LI and L2, acenter tapped inductance 39. Connected between the center tap of the inductance 39 and the common terminal of the condenser 31 and resistor 38, is the primary winding 40, of a transformer I, which has a center tapped secondary winding 40.- The center tap of the winding 40* is connected to the anode 23, while the outer terminals of said winding are connected to the control electrodes 6 and 1, re-

spectively. Smoothing condensers H and 2 are connected across the resistors 29 and 35. respectively.

Upon closure of main switch I3, a circuit extends from the center tap of winding 5" throu h contacts l3", through the windings I0 and 9', armature 2 through the windings 30 and 3|, respectively, to the cathodes 6 and 'I, through the tubes 6 and I, backto the winding 5. Another circuit extends from line LI, through winding I2 contacts Ill and 3 (when closed), to line L2. Another circuit extends from line LI, through winding ii, contacts 9' (when closed), to line L2. There is also a circuit from line Ll, through contacts I2 (when closed), winding 5, .to line L2, and a circuit from line LI, through contacts li (when closed), winding 4, to line L2. A still further circuit extends from line LI through push button switches I4 and I5, upon closure of the latter, through winding I3, to line L2, while a maintaining circuit is provided by the contacts it", which when closed shortcircuits the contacts of switch It.

Before explaining the operation of the system it is desirable to review some of the fundamental operating characteristics of the motors i and 2, for which reference is made to Fig. 2 of the accompanying drawing. It is assumed that the rheostat 3 is provided in each phase with four sections of resistance, thus aflording five speed points. The diagram of Fig. 2 shows five curves marked R1 to R5, respectively, which represent the relation between the speed of the motor I and the torque it is capable of exerting at the five positions of the cross-head.

It will be observed that if the torque. required by the machine is T, and the speed desired is 81, the cross-head 3 must be on the second speed point, that is, the total resistance inserted in the rotor circuit of motor I is R2. Under these conditions the motor 2 is not required to supply any additional torque. If, on the other hand, the speed S1 is required, the motor i will not have sumcient torque to produce that torque at that speed. It is therefore necessary for the motor 2 to supply an additional torque marked A in the diagram. Similarly, is a speed S1" is required, the motor 2 must supply the torque B.

The motor 2 is a separately excited direct current motor, the torque of which, neglecting for the present the armature reaction, is proportional to the armature current. Also the armature voltage of the motor 2, neglecting the resistance drop in the armature, is directly proportional to its speed.

As will be explained more fully hereinafter, the

aforedescribed pilot controller is arranged in such a manner that it responds to the electromotive force induced in the armature of the motor 2. This voltage controls through grids 6 and 1, respectively, the current flow through the corresponding tubes to adjust the current of the armature 2 so that the system will run at the desired speed. The maximum current supplied to motor 2, and therefore the maximum torque aflorded thereby, is limited by the controller, and provision is made for commutating the resistor 3 in such a manner that the motor I supplies the principal component of the total torque, or in other words, that part by which-the total torque exceeds the maximum torque deliverable by the motor 2 as will be explained hereinafter.

The system thus far described operates as follows: Assuming that the machine is at rest and the pilot controller or speed setting device has tacts l3 to maintain the coil l2- energised when the push button switch Ii is released.

Energization of winding II also closes the main contacts l2, l3 and i2, thereby connecting the terminals or the primary winding i to the lines Ll, L2 and L3. with the cross-head 3 in the position shown and the motor I thus energized, maximum resistance is inserted in the 'circuit of the winding l and the motor i tends to operate at its lowest speed. Energization oi the winding II also closes the contacts l3, thereby completing a circuit i'orthe armature 2, and inasmuch u the armature 2' just starts to rotate, the current which it receives is the maximum to which it is limited by its control circuit, for example, 125% oi rated current. The motor 2 therefore exerts its maximum torque, which is added to the torque of motor i to accelerate the machine. The current armature 2 passes through the windings oi relays 3 and It. The relay I is arranged so that it will respond to close contacts 2' when the current of armature 2' is approximately 100% or the normal current and it will respond to open the contacts 2' when the current falls to some lower value. On the other hand, the relay Ills arranged to open the contactg l0 when its energizing current is equal to to oi the normal current of the armature 2', and will close the contacts ll when the magnitude of the current is slightly below this value.

The current supplied to the motor armature 2 through the tubes 8 and 1 is controlled by controlling the potential impressed on the control electrodes F and 1 with respect to their respective cathodes. The potential of the grids 8 and 1 with respect to their cathodes is the resultant or an alternating voltage supplied b winding 40', upon which is superposed the unidirectional potential drop through resistors 29, i9, 22 and 24. The arrangement 01' the circuit is such that the moment of ignition of the tubes 8 and l is advanced to increase the speed of the motor armature 2' when the contact I! is moved toward the bus bar ll. On the other hand, when the current through the resistors 22 and 24 is increased, the resulting voltage drop delays the moment or ignition or the tubes 8 and I, and thus decreases the average current supplied to the motor 2 and thereby its speed is decreased.

The current through the resistors 22 and 24 is controlled through the tubes and 23. These tubes are caused to respond to the current required by the motor armature to compensate for the speed drop or the motor armature due to its resistance. It will be observed that the voltages induced in the windings and II are proportional to the current drawn by the armature 2.

A rectified current, due to said voltages, flows from said windings through the resistors 32 and 3!, back through the rectiflers 33 and II, to the secondary windings 32" and 3|". Thus the volt age drop through these resistors is directly proportional to the armature current.

At the moment before starting the equipment,

no current flows in the circuit or the armature 2. Therefore there is also no voltage drop through the resistor 32. and due to the voltage drop in the resistor 2| the potential of the grid 23 with respect to the cathode 20 is slightly negative. The adjustment is such that some current still flows through the tube 20. The poten .tial ot the grid 23 is zero with respect to its cathode 23'. As a result the tube 23 draws a current which passes through the resistors 22 and 24, to produce a substantial voltage drop therein which impresses a potential on the control electrodes 8 and 1 which is negative with respect to the cathodes 6 and 1 to an extent that the tubes do not pass any current. However, when the starting button it is pressed and the switch it responds, the contacts 13 are opened so that a charging current flows from the bus bar I, through resistor 28, condenser 21, through contacts 3 to bus bar H. The -charge oi the condenser 21 raises the potential of the cathode 23* to make the grid 23 negative with respect to the cathode and the current passed by the tube 23 gradually decreases, thereby decreasing the voltage drop through the resistors 22 and 24, until finally'the potential impressed upon the control electrodes 6 and 1 is sumciently positive to cause ignition of said tubes during the positive halt cycle with resulting passage of current and acceleration of the motor 2. The variation of said grid voltage takes place in a comparatively short time, the purpose being to prevent a too sudden rush of current to the armature. After the motor has accelerated, the drop through the resistor 22 is not sufficient to stop conduction of the tubes 8 and 1.

As the current taken by the armature 2 increases, the voltage across the resistor 32 increases, which causes the grid 20 to become negative with respect to its cathode. This re duces the current through resistor 22 and hence advances the moment of i nition of the tubes 8 and l to permit a higher effective output current to compensate for the voltage drop in the armsture due to the armature resistance. The system is so proportioned that when the armature 2 draws normal full load current, the conduction of the tube 20 is stopped so that any further increase in the armature current would have no eilect on the regulation effect of said tube. However, the voltage drop across the resistor 35 is increased at the same time and this voltage is applied to the grid 23, which is normally negative with respect to the cathode 23, as has been explained heretofore. The circuit is so arranged that when the current in the armature circuit reaches the allowable maximum, the tube 23 becomes conducting which causes a voltage drop through the resistors 22 and 28, to thereby retard the moment of ignition of the tubes '6 and l.

The system provides for an additional compensating voltage component which is proportional to the armature. voltage and therefore the speed. The resistor 29 is connected across the armature 2, and therefore the potential of the movable contact 29 for any setting is directly propos tional to the armature voltage. For this purpose the contact 29' is connected to the contact I9, 01 the voltage divider I9. The contact 29 is normally set to apply a given portion of the armature voltage to the grids 6 and 1 and this voltage is balanced out by the drop in the resistor l9 when the motor operates at the speed corresponding to the setting of contact I9. If the motor rims with a given load and the load tends to increase, the armature voltage will tend to decrease. This decreases the voltage drop in resistor 23 until a new balance is obtained through the action of the regulator which causes the motor speed to increase again until the voltage drop in the resistors and I9 are alike.

It will be apparent that as long as the speed of the system is much below that for which the resistor i9 is adjusted, the motor 2 operates with its full load current, while motor supplies additional torque.

As a result of the high value of the armature current, the contacts 9 are closed, but the contacts H1 are opened. This causes energization of the relay II, which in turn closes its contacts li thereby supplying powerthrough the winding 4 of the motor 4 and causing the pilot motor 4 to rotate to move the cross-head 3 in a direction to cut out part of the resistors 3 3 and 3.

As the speed approaches the value for which the pilot controller is set, the motor may tend to take all of the load, with the result that the current taken by the armature 2- is decreased by action of the tubes 6 and I, and the contacts 9 are opened, thereby deenergizing relay. I, which in turn disconnects pilot motor 4 to stop further reduction of resistor 3. The motors I and 2 now divide the load between each other in accordance with their respective characteristics. If later the load on the system decreases, the motor tends to accelerate above the desired speed. This results in a decrease of the current of the armature 2 which, if great enough, will make the current LI and L2. A condenser 51 is connected in shunt with the winding 52. The cathode 5551s connected to the common terminal of the-:resistors 53 and 54, and to the remaining end tap-or winding 56, while the grid 55 is connected in series with a limiting resistors 58, to the common point of the armature 2- and the resistor 54. A relay 59 is provided, having an energizing winding 59 and normally open contacts 59 In addition to the connections of the elements which are the same as those of the system Fig. 1,

there are the following further connections: A

circuit extends from line Ll, through the contacts 52, energizing winding 5|, contacts l3, when closed, to line L2. Another circuit extends from line L|,'through contacts 52, when closed, winding through contacts I3 when closed, to line L2. The cathode is connected to one terminal of each of the contacts 50* and 5 l. One terminal of each of the contacts 5|! and 5| is connected to the center tap of the winding 5'.

One terminal of each of contacts 50 and 5| is -connected to the common terminal of the re-' sufficiently low to permit closure of the contacts 19 of the relay l0, whereby the relay coil |2 is energized, causing closure of the contacts I2", which in turn energizes the motor winding 4. This causes the pilot motor 4 to operate in the reverse direction to move the cross-head 3, to increase the amount of resistance inserted by the resistors 3, 3 and 3 in the circuit of the armature l This action of the resistor 3 causes a decrease of the speed of the machine with a corresponding increase of the current taken by the armature 2 to a value which will maintain the speed of the machine in accordance with the adjustment of the pilot controller.

Referring now to Fig. 3, the same shows a modification of Fig. 1, in which the circuit connections of the auxiliary motor 2 are reversible to produce a torque either in the same direction as the motor I, or in opposition thereto, depending upon whether the speed which obtains with a open auxiliary contacts 5|] and 5|, respectively.

An electromagnetic relay 52 is added which has an energizing winding 52 normally open contacts 52 and normally closed contacts 52. Connected in series with each other and across the armature 2 are resistors 53 and 54. A gaseous electron tube 55 is provided, having a cathode 55 an anode 55 and a control grid 55. The winding 52 is connected between the anode 55 and one end tap of the secondary winding 56 of an isolating transformer 56. The transformer 58 has a primary winding 56, connected to the lines sistor 53 and the armature 2, while one terminal of each of the contacts 50 and 5| is connected through the winding 59, transformer windings 3-0 and 3| to the respective cathodes 6 and 1 A circuit also extends from line Ll, through limit switch 3 when closed, through contacts 5|, winding 82*, contacts 59*, when closed, to line L2, and another circuit from line Ll, through contacts 50 winding contacts 59, when closed, to line L2. A further circuit extends from limit switch 3, through contacts |3 when closed, to the terminal of winding 4, which is also connected to the contact I2 The operation of this modified system shown in Fig. 3 is as follows: After the rheostat |9 has been adjusted to the speed at which it is desired to operate, the starting button I5 is depressed, which energizes the coil l3 as has been explained heretofore, thereby energizing the motor The potential of grid 55 is zero with respect to cathode 55, so that tube 55 conducts current, which energizes coil 52, to close contacts 52*, thereby establishing a circuit from line Ll, through contacts 52 coil 50, contacts l3 to line L2. This energizes the forward switch 50. The interrupted unidirectional current flowing to the coil 52 during alternate half-waves of the alternating current supply is smoothed out by the condenser 51 in the usual manner, to maintain energization of the coil during the half-cycle when the tube 55 is non-conducting. Response of relay 52 also opens the contacts 52, thereby deenergizing the switch 5|. The closing of contacts 50 and 50 connects the armature 2 in series with the tubes l 6 and 1 to the transformer winding 5*, and causes the motor 2 to supply a positive torque. The arrangement is such that the current flow in the armature 2 is of the order of of normal. The relay 59 responds to the armature current, closing the contacts 59". This'completes the circuit for the winding of the relay H, thereby energizing the winding 4 of the pilot motor 4, causing it to rotate to reduce the resistor 3 which accelerates the motor I. Ultimately the crosshead 3 will have cut out suflicient resistance so that the speed of the motor is approximately that for which the pilot speed controller is set. When this approximate speed is attained, the current of the motor 2 is reduced to a value which ultimately causes sufiicient deenergization of the relay coil 59 and the relay 59 opens to stop the motor 4 and prevents further reduction oi the resistance of resistor 3.

As long as the motor 2 supplies a positive torque, that is, a long as the torque of motor i alone is insuilicien-t to aflord the desired speed, a current flows through the resistor 54 in such a direction that the voltage drop through the latter causes the potential of the grid 55 to tend to become positive with respect to the cathode 55. The grid 55 is prevented from becoming positive only by the action of grid resistor 58. Under this condition tube 55 is conducting, relay 52 remains energized, and relay 50 is energized through contacts 52". If now the speed of the motor I should become suiliciently higher than that for which the speed regulator is set, the motor 2 is driven as a generator so that current in this case flows from the terminal of the armature 2 marked through the resistors 53 and 54 and back to the terminal marked This makes the grid 5.": negative with respect to the cathode 55, so as to cause tube 55 to become non-conducting and relay 52 to become deenergized. This causes energization of relay through contacts 52 and reversal of the connections of the armature 2 with respect to the current supply, so that the torque of the motor 2 and the polarity of the armature are reversed, the motor thereby exerting a braking enect.

In the arrangements illustrated in each of Figs. 1 and 3 the pilot motor 4 remains at rest unless the current of motor armature 2 reaches a sufficiently high value, say 100% of rated value. The result of attaining such a. high value, in Fig. 3, for example, is to cause closure of contacts 59* by energization of coil 59". In this event the pilot motor runs in one direction or the other, depending upon whether contacts 50' or Si are closed, until the current has dropped sufficiently to drop out relay 59. In all speed and torque conditions between those which cause energization of relay 59, the auxiliary motor 2 provides stepless control of speed and supplies suflicient positive or negative torque at all times to maintain the preset speed in spite of changes in the load torque. The characteristics of the system of Fig. 1 in this respect will be apparent from the foregoing description.

I claim:

1. In a system for obtaining and maintaining a preselected speed of a prime mover subjected to a varying load, in combination, a prime mover, means to supply power to said prime mover, means to regulate such power between given limits in a predetermined number of equal steps, an auxiliary dynamo electric machine coupled to said prime mover to coact with said prime mover and automatic control means for said dynamo electric machine, comprising, a source of energy supply for said dynamo electric machine to afford a flow of energy between said source and said dynamo electric machine in varying amounts according to departures of the speed of said prime mover from the preselected speed, and means responsive to flow of energy between said dynamo electric machine and said energy source but only when in excess of the power change afforded by one of said steps, to effect adjustment of said power regulating means, following gradual return of said motor toward the preselected speed by the coaction of said dynamo electric machine, whereby saidpower regulating means and said dynamo electric machine jointly provide smooth continuous speed regulation.

2. In a system for obtaining and maintaining a preselected speed of a motor subjected to a varying load, in combination, a motor, means to supply electric power to said motor, means to regulate such power between given limits in a predetermined number oi equal steps, an auxiliary dynamo electric machine coupled to said motor to coact with said motor and automatic control means for said dynamo electric machine, comprising, a source of energy supply for said dynamo electric machine to aflord a flow of energy between said source and said dynamo electric machine in varying amounts according to departures of the speed of said motor from the preselected speed and means responsive to flow or energy between said dynamo electric machine and said energy source but only when in excess of the power change afforded by one of said steps, to efiect adjustment of said power regulating m following gradual return of said motor toward the preselected speed by the coaction of said dynamo electric machine, whereby said' power regulating means and said dynamo electric machine jointly provide smooth continuous speed re ulation.

3. In a system for obtaining and maintaining a preselected speed of a motor subjected ,to a varying load, in combination, a motor, means to supply electric power to said motor, means to regulate such power between given limits in a predetermined number of equal steps, an auxiliary dynamo electric machine coupled to said motor to coact with said motor and automatic control means for said dynamo electric machine,

comprising, a source of energy supply for said dynamo electric machine to afiord a ilow of energy between said source and said dynamo electric machine in varying amounts according to departures of the speed of said motor from the preselected speed and means responsive to flow of energy between said dynamo electric machine and said source but only when in excess of the power change afforded by one of said steps, to eifect adjustment of the step by step regulating means fOllOWlIig gradual return of said motor toward the preselected speed by the coaction of said dynamo electric machine, whereby said speed regulating means and said dynamo electric machine jointly provide smooth continuous regulation.

4. In a system for obtaining and maintaining a preselected speed of a motor subjected to a varying load, in combination, a motor, means to supply electric power to said motor, means to regulate such power between given limits in a predetermined number of equal steps, an auxiliary dynamo electric machine coupled to said motor to assist said motor and automatic control means for said dynamo electric machine, comprising, a source of energy supply for said dynamo electric machine from which it draws energy in varying amounts according to departures of. the speed of said motor from the preselected speed and means responsive to flow of energy between said dynamo electric machine and said source but only when in excess of the power change afforded by one of said steps, to eifect adjustment of the step by step regulating means following gradual return of said motor toward the preselected speed by the assisting action of said dynamo electric machine, whereby said means and said dynamo electric machine jointly provide smooth continuous speed regulation.

5. Ina system for obtaining and maintaining a preselected speed of a motor subjected to a varying load, in combination, a, motor, means to supply electric power to. said motor, means automatic control means for said dynamo electric machine, comprising, a source of energy supply for said dynamo electric machine from which it draws or delivers energy in varying amounts according to" departures of the speed of said motor from the preselected speed and means responsive to flow of energy between said dynamo electric machine and said source but only when in excess of the power change afforded by one of said steps, to effect adjustment of the step by step regulating means following gradual return of said motor toward the preselected speed by the assisting or bucking action of said dynamo electric machine, whereby said speed regulating means and said dynamo electric machine jointly provide smooth continuous regulation.

6. In a system for obtaining and maintaining a preselected speed of a motor subjected to a varying load, in combination, a motor, means to supply electric power to said motor, means to,

dynamo electric machine to afiord a flow of energy between said source and said dynamo electric machine in varying amounts according to departures of the speed of said motor from the preselected speed and means responsive to flow of energy between said dynamo electric machine and said source but only when in excess of the power change afforded by one ofsaid steps, to effect adjustment of the step by step regulating means following gradual return of said motor toward the preselected speed by the coaction of said dynamo electric machine, whereby said speed regulating means and said dynamo electric machine jointly provide smooth continuous regulation, said last mentioned means including means to limit such flow of energy to a given maximum.

7. In a system for obtaining and maintaining a preselected speed of a motor'subiected to a regulate such power between given limits in a predetermined number of equal steps, an auxili- 12 tioned means including means to limit said flow of current to a given maximum.

8. In a system for obtaining and maintaining a preselected speed of a motor subjected to a varying load, in combination, an induction motor, means to supply electric power to said motor, means to regulate such power between giv n limits in a predetermined number of equal ste s, an auxiliary dynamo electric machine coupled c said motor to selectively assist or buck said mot r and automatic control means for said dynamo electric machine, comprising, a source of alternating current, electronic means connected between said dynamo electric machine and said ary dynamo electric machine coupled to said" motor to coact with said motor and automatic control means for said dynamo electric machine, comprising, a source of alternating current, electronic means connected between said dynamo electric machine and said source to aflord a flow of rectified current therebetween in amounts varying according to departures of the speed of said motor from the preselected speed and means responsive to flow of current between said dynamo electric machine and said source but smooth continuous regulation, said last mensource toafiord a flow of rectified current therebetween in amounts varying according to departures of the speed of said motor from the preselected speed and means responsive to flow of current between said dynamo electric machine and said source but only when in excessof the power change afforded by one of said steps, to effect adjustment of the step by step regulating means following gradual return of said motor toward the preselected speed by the coaction of said dynamo electric machine, whereby said speed regulating means and said dynamo electric machine jointly provide smooth continuous regulation, said last mentioned means including means to limit said flow of current to a given maximum.

9. In a system for obtaining and maintaining a preselected speed of a motor subjected'to a varying load, in combination, an induction motor,

electric machine and said source to afford a flow of rectified current therebetween in amounts varying according to departures of thespeed of said motor from the preselected speed and means responsive to flow of current between said dynamo electric machine and said source but only when in excess of the power change afiorded by one of said steps, to efiect adjustment of the step by step regulating means following gradual REFERENCES CITED The following references are of record inthe file of this patent:

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